Dear Dave: Clarification: I wasn’t using shot. I was using abrasive
media, which contains some kind of quartz powder in a carrier of
plastic. It does produce a residue, which has to be washed off
thoughly before drying and storing the media. The media wears down a
little with each load run, leaving a sludge or slurry. I dump the
tumbler load into a strainer and rinse both media and workpieces,
then separate and wash both again as many times as necessary. It’s
really a kind of pre-polish cycle.
I agree that steel shot should not produce residue (unless you’re an
awful slob), since it’s peening and burnishing, rather than
polishing. Having once worked in a research lab with dead animals and
noxious chemicals, I’m an obsessive washer of everything (at least
everything work-related). I see contamination everywhere.
In the case I mentioned with the etched pieces, the lines were so
fine I never noticed that the sludge had settled into the lowest
parts. I had scrubbed, as usual, with a toothbrush during rinsing. A
wet dark line looks like a wet dark line. I speculate that the sides
and bottom of the etched lines had a micro-texture that held the
particulate sludge. Not until it dried a lighter color (medium grey)
did I realize that the lines were not clean.
Someday when I have time I might exploit this effect…call it “easy